Index-coupled distributed-feedback semiconductor quantum cascade lasers fabricated without epitaxial regrowth

Abstract

Quantum cascade (QC) lasers and methods of fabricating such QC lasers are provided. The QC lasers incorporate a DFB grating without requiring the use of relying on epitaxial regrowth processes. The DFB gratings are formed as sidewall gratings along the lateral length of the QC active region, or the DFB gratings are formed atop the lateral length of the QC active region, and wherein the top DFB grating is planarized with a polymeric material.

What is claimed is: 
     
       1. A quantum cascade laser comprising:
 an elongated waveguide ridge having characteristic lateral dimensions comprising a height and a width, and a characteristic longitudinal dimension comprising a length, the waveguide ridge being formed of a plurality of epitaxial layers, the lateral and longitudinal dimensions of the waveguide ridge defining a laser cavity comprising an active region of semiconductor quantum well structures configured to employ intersubband electronic transitions; 
 at least one conductive epitaxial cladding layer disposed atop and below the active region of the waveguide ridge; 
 a distributed feedback grating comprising a plurality of periodic vertical corrugations formed into both lateral width edges of the waveguide ridge along the longitudinal length of said waveguide ridge, the corrugations having characteristic modulation depth and pitch; 
 a dielectric layer conformally disposed atop the distributed feedback grating; 
 a conductive contact layer disposed atop the dielectric layer and elongated waveguide ridge; 
 wherein the corrugations are disposed through at least the top cladding layer and each of the epitaxial layers of the waveguide ridge; and 
 wherein the lateral dimensions along the length of the waveguide ridge are comparable to or less than the wavelength of an emission confined therein to impose a single lateral spatial mode thereon; and 
 wherein the modulation depth and pitch of the corrugations are configured such that the corrugations have a coupling coefficient sufficient to interact with the active region of the laser to impose a single spectral mode on an emission confined therein. 
 
     
     
       2. The quantum cascade laser of  claim 1 , wherein the corrugations have an open profile having a modulation depth, d, that is equal to or less than the pitch, Λ, of the periodic vertical corrugations such that the aspect ratio of the corrugations, d/Λ is less than 1. 
     
     
       3. The quantum cascade laser of  claim 1 , wherein the corrugations are configured such that no tangent of the corrugation surface is perpendicular to the longitudinal axis of the waveguide ridge. 
     
     
       4. The quantum cascade laser of  claim 1 , wherein the dielectric layer is formed of a material at a thickness sufficient to isolate the optical mode guided by the laser waveguide from the conductive contact layer;
 wherein the dielectric material is transparent at the laser emission wavelength; and 
 wherein the dielectric material has a lower refractive index than the effective index of the laser active region thereby confining light to the laser ridge. 
 
     
     
       5. The quantum cascade laser of  claim 1 , wherein the dielectric layer is configured to allow for the conduction of thermal energy from the active region into the contact layer. 
     
     
       6. The quantum cascade laser of  claim 1 , wherein at least one of the top and bottom cladding layers are formed from InP, wherein the epitaxial layers of the waveguide ridge are formed from a combination of InGaAs and AlInAs, wherein the dielectric layer is formed of an AlN and/or SiN x  material having a thickness of less than 1 μm, and wherein the conductive contact layer is formed from metal having a thickness of at least 2 μm. 
     
     
       7. A quantum cascade laser comprising:
 an elongated waveguide ridge having characteristic width and length dimensions and being formed of a plurality of epitaxial layers, the longitudinal length of the waveguide ridge defining a laser cavity comprising an active region of semiconductor quantum well structures configured to employ intersubband electronic transitions; 
 at least one epitaxial cladding layer disposed below and atop the active region of the waveguide ridge; 
 a distributed feedback grating comprising a plurality of corrugations formed into the top cladding layer across a portion of the width of the waveguide ridge along the longitudinal length of said waveguide ridge, the corrugations having characteristic modulation depth and profile; 
 a polymeric planarization infill layer disposed atop the distributed feedback grating to form a smooth top surface; 
 a dielectric layer disposed on the sidewalls of the waveguide ridge; and 
 a conductive contact layer disposed atop the dielectric layer and elongated waveguide ridge; 
 wherein the width is configured such that the waveguide ridge supports a single lateral spatial mode; and 
 wherein the modulation depth and profile of the corrugations are configured such that the corrugations have a coupling coefficient sufficient to interact with the active region of the laser to impose single-mode operation and emission at specific engineered wavelengths thereon. 
 
     
     
       8. The quantum cascade laser of  claim 7 , wherein the dielectric layer is formed of a material at a thickness sufficient to isolate the optical mode guided by the laser waveguides from the contact layer. 
     
     
       9. The quantum cascade laser of  claim 7 , wherein the dielectric layer is formed to allow for the conduction of thermal energy from the active region into the contact layer. 
     
     
       10. The quantum cascade laser of  claim 7 , wherein the polymeric planarization infill layer comprises a polymer with a refractive index less than the epitaxial top cladding layer. 
     
     
       11. The quantum cascade laser of  claim 7 , wherein at least one of the top and bottom cladding layers are formed from InP, wherein the epitaxial layers of the waveguide ridge are formed from a combination of InGaAs and AlInAs, wherein the dielectric layer is formed of an AlN and/or SiN x  material having a thickness of less than 1 μm, and wherein the conductive contact layer is formed from metal having a thickness of at least 2 μm.